The electrical activity of the nervous system depends on the activities of diverse ion channels. Specialized Na+ and K+ channels generate and shape the nerve impulse, which has a characteristic rising phase resulting from an inward Na+ current, and a falling phase resulting from an outward K+ current. Kv3.4 is a voltage-gated K+ channel that shapes the falling phase of the nerve impulse in a manner that depends on its phosphorylation state. Kv3.4 channels autoregulate their function by undergoing fast inactivation, which is dramatically modulated by phosphorylation. Fast Kv3.4 inactivation is eliminated upon protein kinase C (PKC)-dependent phosphorylation of four serines (8, 9, 15 and 21) within the N-terminal inactivation domain. Although the mechanism of this modulation is well understood, its role and regulation in native systems under physiological and pathological conditions are unknown. Additionally, we found that the Kv3.4 current is down-regulated in dorsal root ganglion (DRG) neurons upon persistent PKC activation as well as in a model of spinal cord injury (SCI). We hypothesize that Kv3.4 exists in a signaling complex that tightly regulates inactivation as well as functional expression and that this regulation is altered in SCI. To investigate this hypothesis in DRG neurons, we plan to pursue the following specific aims: 1. To investigate novel Kv3.4 phosphorylation sites possibly implicated in channel down-regulation induced by PKC and SCI. 2. To investigate the molecular components of the putative Kv3.4 signaling complex in DRG neurons. To accomplish these aims, we plan to combine electrophysiological, immunological, biochemical and molecular biology techniques. These aims will break new ground by identifying novel mechanisms of Kv3.4 modulation in sensory neurons, and shedding light on the molecular underpinnings of the hyperexcitable state responsible for the persistent pain associated with SCI.